Monica Calatayud

Adsorption on perfect and reduced surfaces of metal oxides

Two major chemical processes, acidobasic and redox, track the adsorption mechanism on metal oxides. Molecular and dissociative adsorption on stoichiometric surfaces can be understood as acid–base processes. Clean and anhydrous surfaces of metal oxides have two different active sites: cations and anions. Electron-rich molecules or fragments arising from a heterolytic bond cleavage (Lewis bases) react with Mn+, while electron poor ones (Lewis acids) react with O2−. The MgO and TiO2 surfaces clearly appear to be predominantly acidic and molecules that do not dissociate generally bind to the metal cation. The electronic structure, insulating character for the stoichiometric surface, is preserved upon adsorption. When the initial system does not favor an energy gap (open-shell adsorbates, defective surfaces), the best adsorption mode is via a redox mechanism that restores the situation of an insulator and the highest oxidation states for all the atoms. For radical adsorption a first solution occurring on irreducible oxides is to couple the electrons and form two opposite ions adsorbed on the two surface sites, as for H2/MgO, involving an acid–base mechanism. Another possibility occurring on reducible oxide is via an electron transfer to or from the oxide (redox mechanism). The electron transfer occurs from the substrate to the adsorbate for electronegative group (Cl adsorption on O) or the other way around for an electropositive one (NO adsorption on M). The reactivity at surfaces deviating from stoichiometry differs from that on the perfect ones. The difference does not only originate from the modification of the coordination number but also from the electron counting.

Theoretical study on the role of surface basicity and lewis acidity on the etherification of glycerol over alkaline earth metal oxides.

Alkaline earth metal oxides (MO) are catalytically active in the etherification of glycerol. Density Functional Theory (DFT) calculations have been used to examine the reactivity of glycerol with MO surfaces with M=Mg, Ca, Sr or Ba. More specifically, the optimum glycerol adsorption mode and the strength of glycerol interaction with regular MO (001) surfaces and a stepped CaO surface have been investigated and involves the interaction with acid-base surface sites. The basicity of lattice oxygen atoms is correlated with the adsorption energy: BaO (-3.02 eV) > SrO (-2.85 eV) > CaO (-2.05 eV) > MgO (-1.35 eV). The interactions have an exothermic character, that is, the more basic the alkaline earth metal oxide, the more exothermic is the adsorption process and the higher the dissociation extent. Thus, the dissociation of glycerol increases in the order: MgO (not dissociated) < CaO (partially dissociated < SrO (partially dissociated) < BaO (completely dissociated). The presence of defects is found to play a key role in the mechanism: glycerol interaction with a stepped CaO surface presents the highest adsorption energy (-3.78 eV), and the molecule is found to dissociate at the step. The calculated structural parameters are found to be in good agreement with experimental data on catalyst reactivity. Moreover, the earlier postulated reaction mechanism, which also involves the additional involvement of Lewis acid sites proved to be feasible for CaO and SrO regular surfaces, and for the stepped CaO surface. It was found that for these oxides one of the most favored adsorption modes involves a non-dissociative adsorption of one hydroxyl group of glycerol, which as a result becomes a better leaving group. Therefore, theoretical evidence was found for the possible direct involvement of Lewis acid sites in the catalytic etherification of bio-derived alcohols, such as glycerol, as it is anticipated that these observations can be extended to sugar alcohols as well.

Stability of formate species on β-Ga2O3

Gallia (gallium oxide) has been proved to enhance the performance of metal catalysts in a variety of catalytic reactions involving methanol, CO and H(2). The presence of formate species as key intermediates in some of these reactions has been reported, although their role is still a matter of debate. In this work, a combined theoretical and experimental approach has been carried out in order to characterize the formation of such formate species over the gallium oxide surface. Infrared spectroscopy experiments of CO adsorption over H(2) (or D(2)) pretreated beta-Ga(2)O(3) revealed the formation of several formate species. The beta-Ga(2)O(3) (100) surface was modelled by means of periodic DFT calculations. The stability of said species and their vibrational mode assignments are discussed together with the formate interconversion barriers. A possible mechanism is proposed based on the experimental and theoretical results: first CO inserts into surface (monocoordinate) hydroxyl groups leading to monocoordinate formate; this species might evolve to the thermodynamically most stable dicoordinate formate, or might transfer hydrogen to the surface oxidizing to CO(2) creating an oxygen vacancy and a hydride group. The barrier for the first step, CO insertion, is calculated to be significantly higher than that of the monocoordinate formate conversion steps. Monocoordinate formates are thus short-lived intermediates playing a key role in the CO oxidation reaction, while bidentate formates are mainly spectators.

Ionization of HCl and HF in ice: a periodic DFT study

Abstract We present the results of periodic DFT–GGA calculations on the HCl and HF ionization inside a cubic ice structure. The model considers the substitution of a water molecule by an HX molecule. The potential energy curves are drawn at fixed H–X distances. HCl dissociation inside the ice structure is a barrierless process, while HF presents a typical dissociation potential curve. The selected computational methodology is able to reproduce the experimental observations.

Theoretical Investigation of NO Oxidation over TiO2-Anatase

We present periodic DFT calculations to study the NO adsorption on stoichiometric (100) and (001) TiO2-anatase surfaces. The adsorption on the Ti atoms, involving an oxidation of the substrate, is very weak. The adsorption on the O atoms, involving a reduction of the substrate, is possible when these atoms are in terminal position. Then, nitrite and nitrates can be formed. The adsorption energy for their formation on an oxygen-terminated slab model is ten times larger than the molecular NO adsorption on a titanium atom.

Density functional theory study of the oxidation of methanol to formaldehyde on a hydrated vanadia cluster

Density functional theory was used to study the mechanism for the oxidation of methanol to formaldehyde. A vanadium oxide cluster OV(OH)3 has been utilized to represent the catalytic system under hydrated conditions, i.e., in the presence of VOH hydroxyl groups. Two types of methoxy‐intermediates have been considered: a penta‐coordinate methoxy‐intermediate (OH)4V(OCH3) and a tetrahedral methoxy‐intermediate (OH)2VO(OCH3)(H2O). The most plausible reaction pathway corresponds to the process involving first the formation of the tetrahedral methoxide, and a subsequent rate‐limiting step where hydrogen is transferred from the methoxy groups toward the oxygen atom of the vanadyl VO site. The reaction mechanism is a typical two‐state reactivity process due to a change of the multiplicity (reactive singlet → product triplet) along the reaction coordinate accompanied by a reduction of the vanadium center from VV (d0) to VIII (d2). Minimum energy crossing points were localized and possible spin inversion processes are discussed by means of the intrinsic reaction coordinate approach to find the most favorable reaction pathways. The hydration effect is found to be mainly the destabilization of the methoxy intermediates. An alternative reaction pathway with a lower apparent barrier is presented.

Probing Raman enhancement in a dopamine-Ti2O4 hybrid using stretched molecular geometries.

Hybrids consisting of a metal oxide nanoparticle and a molecule show strong enhancement of Raman modes due to an interfacial charge transfer process that induces the formation of midgap states, thereby reducing the effective gap compared to that of the nanoparticle and creating the posibility of an electronic resonance at energies substantially lower than the nanoparticless band gap. We have developed a simple methodology to mimic the presence of the nanoparticle through a deformation of the bond involved in the chemical binding between the two entities forming the hybrid. The results provide a convincing interpretative frame to the enhancements observed in Raman spectra when all atoms are included. In addition, these enhancements can be correlated to a crossing of excited molecular orbitals that take part in the virtual excitation associated with the Raman process. We illustrate our method for the dopamine-Ti2O4 hybrid using the most acidic molecular O-H bond as the control parameter for the deformation.

Toward an understanding of the hydrogenation reaction of MO2 gas-phase clusters (M = Ti, Zr, and Hf).

A theoretical investigation using density functional theory (DFT) has been carried out in order to understand the molecular mechanism of dihydrogen activation by means of transition metal dioxides MO2 (M = Ti, Zr, and Hf) according to the following reaction: MO2 + H2 → MO + H2O. B3LYP/6-311++G(2df,2pd)/SDD methodology was employed considering two possible reaction pathways. As the first step hydrogen activation by M═O bonds yields to metal-oxo hydride intermediates O═MH(OH). This process is spontaneous for all metal dioxides, and the stability of the O═MH(OH) species depends on the transition metal center. Subsequently, the reaction mechanism splits into two paths: the first one takes place passing through the M(OH)2 intermediates yielding to products, whereas the second one corresponds to direct formation of the product complex OM(H2O). A two-state reactivity mechanism was found for the TiO2 system, whereas for ZrO2 and HfO2 no spin-crossing processes were observed. This is confirmed by CASSCF/CASPT2 calculations for ZrO2 that lead to the correct ordering of electronic states not found by DFT. The results obtained in the present paper for MO2 molecules are consistent with the observed reactivity on surfaces.

Cation distributions on CoAl2O4 and Co2AlO4 spinels: pressure and temperature effects

Structural, energetic and thermodynamic properties of CoAl2O4 and Co2AlO4 spinels are investigated using the density functional theory formalism. Thermal effects are incorporated by means of a non-empirical quasi-harmonic Debye-like model that allows us to study the influence of temperature on the relative stability of different cation distributions over tetrahedral and octahedral interstices of the oxygen sub-lattice. Our simulations are able to reproduce the experimentally observed trend of preferring a greater cationic disorder in CoAl2O4 than in Co2AlO4. This behavior can be explained as due to the higher average charge (smaller size) of the cobalt ion in Co2AlO4 (+2.5) than in CoAl2O4 (+2). According to our calculations, pressure and temperature enhance the preference for structures with Al3+ in tetrahedral sites (and Co2+ at octahedral sites).

Scaling reducibility of metal oxides

The reducibility of bulk metal oxides in which the cation is in its highest oxidation state (MgO, Sc2O3, Y2O3, TiO2, m-ZrO2, m-HfO2, CeO2, V2O5, Nb2O5, Ta2O5, WO3, CrO3, Al2O3, β-Ga2O3, SiO2, SnO2 and ZnO) has been studied by standard periodic density functional theory. We have defined and calculated descriptors able to describe and quantify semi-quantitatively the extent of reduction: electronic band gap, oxygen vacancy formation energy and electronic localization. We find that there is no single criterion for characterizing the reducibility. We discuss the advantages and limitations of each method, and we apply them to classify the materials with the PBE+U and B3LYP functionals. Typical irreducible oxides such as MgO show a large band gap, high oxygen vacancy formation energy and electronic localization of the reduction electrons forming and F-center, with a diamagnetic singlet electronic state. Reducible oxides such as TiO2 present small band gaps, small oxygen vacancy formation energy and electron localization of the reduction electrons in the cations, decreasing their oxidation state and presenting open-shell electronic states. Intermediate or ambivalent behavior is found for ZrO2, HfO2, β-Ga2O3, ZnO and SnO2.